For producing packs by packing items in packagings, there are already commercially available machines in which, for example, recessed packagings filled with items are closed and sealed with a top sheet. In this way, vacuum packs or packs with an inert gas filling can be created.
When packaging certain items of food, for example cheese, fruit and vegetables such as asparagus or lettuce and other items of food or similar products that need to breathe, closed packagings have the disadvantage that gases are produced in the pack during the maturing or ripening process, in particular ethylene and carbon dioxide, and these have an adverse effect on the shelf life. Ethylene in particular speeds up the maturing or ripening process and then shortens the shelf life. One of the effects of carbon dioxide is to cause undesired fermentation processes, and is consequently also detrimental to the shelf life. For such items to have a good shelf life, an atmosphere that is adapted to the product to be packed is desirable, for example in the case of certain items of food an oxygen content that remains as constant as possible in the atmosphere inside the packs, for example at approximately 5%, while undesired gases such as carbon dioxide and ethylene are removed. For this purpose, a change has taken place in favor of using appropriately perforated films or sheets. The top sheets concerned can usually be obtained by the operator of the packaging machine in rolls from film and sheet suppliers.
In the document DE 10 2006 017 887.4, which is not a prior publication, a description is also given of a packaging machine in which a perforating unit is provided for perforating the top sheet. As a result, the same sheet that is used for tightly closing the packs can be used for packs with a selectively permeable top membrane.